Hi,
Sounds like the taper needle bearings are dry/worn or more likely both. Someboby else had a similar query a couple of weeks ago?. It's fairly common.
These are the bearings mounted on the two pivot pins immediately in front of the gaitor you removed. However I'm not sure why the noise should disappear then return when torque arm is slackened then tightened?, but I do know the creak from the pivot bearings is usually worse when bevel box warm after a run - ie b/box expanded slightly due to heat but s/arm still cool, so pre-load is increased?
Reference torque arm -- please note this should only be tightened when the suspension is loaded to it's normal running height - ie off stand and heavy rider sitting on bike and tighten in this position. The movement in this joint is only provided by "twisting" the rubber in the Metalastic-type joint fitted here so this is important so ensure rubber is twisted slightly in both directions from normal ride-height rather than all in one direction from c/stand height.
If you wanting to change these bearings yourself I think I've got a "layman's" guide (ie no B-W special tools etc) which I got from the Motorad S.A. web site.
Cheers..............................Grizzly